Transition metal oxide nanowires (nanowires), such as titanium oxide, cobalt oxide, manganese oxide, and tungsten oxide nanowires, are being increasingly utilized in emerging renewable energy technologies such as solar cells [1], lithium ion batteries [2, 37-40], capacitors [41-43], catalysts [44-47], and composites [48]. Each of these applications requires amounts of nanowire materials on the order of several hundred grams for testing and device prototyping. However, current synthesis methods for producing these nanowires are generally only capable of producing a few milligrams in a single batch, thus making the production of sufficient amounts of the transition metal nanowires both time and labor intensive.
Traditionally, liquid-phase hydrothermal methods, which employ high-pressure conditions and multiple steps, have been used to synthesize the nanowire materials [9, 49]. Such hydrothermal methods can result in small amounts of products (1 gram/day) in a batch mode, but the hydrothermal technique itself is generally slow, with reactions occurring over several hours to days, and is thus unsuitable for industrial applications. Two recent modifications to the hydrothermal technique are microwave-hydrothermal [50, 51] and continuous flow hydrothermal methods [52]. These additional techniques have increased the production rate of nanowires to about 5 g/day and 10 g/hr, respectively, with the microwave hydrothermal technique also using microwave radiation energy instead of electrical heating to improve the synthesis time. Nevertheless, the microwave hydrothermal technique has still not been found to be suitable for producing sufficient quantities of transition metal oxide nanowires for testing and device prototyping, and the continuous flow hydrothermal method has been shown to work only for nanoparticles (NPs) and not nanowires. Similarly, other synthesis methods, including electro-deposition [53], sol-gel [54], as well as several other methods, have only been shown to synthesize transition metal oxide nanowires on a milligram scale.
Recently, it has been observed that low-melting metals can be oxidized directly to produce respective metal oxide nanowires [13]. Indeed, such schemes were implemented with thermal oxidation in a horizontal tubular reactor for Zn [55], Sn [56], Ga [57], In [58], and certain high melting metals, such as W [59], Ta [59], Mo [60]. Additionally, another scheme using direct plasma oxidation of certain metals, such as Fe [16], Nb [15], and V [15], to synthesize metal oxide nanowires onto metal foils has also been used. Nevertheless, gas phase synthesis of these transition metal oxide nanowires without any external reagents has proven difficult because of the high melting points of their respective metals and the tendency of the metals to oxidize rapidly, which makes it difficult to melt the metals without the concurrent formation of an oxide vapor plume that then limits the amount of transition metal oxide nanowires that can be obtained.
In any event, known methods of synthesizing transition metal oxide nanowires are only capable of producing small quantities of transition metal oxide nanowires or are only capable of producing transition metal oxide nanowires with the undesirable formation of an oxide vapor plume. Furthermore, none of the known methods address how to produce a sufficient amount of transition metal oxide nanowires in a short amount of time, which is of great importance in producing transition metal oxide nanowires for industrial applications.